1. Field of the Invention
The present invention relates to converters for hearing aids used in the rehabilitation of damaged inner ears. In particular, the present invention relates to such converters for hearing aids which are hermetically sealed.
2. Description of the Related Art
Hearing aids for rehabilitating damaged inner ear typically pick up sound with a microphone and using this microphone, convert the sound into an electrical signal. This signal is processed in analog or digital form by an electronic unit and is amplified. The amplified electrical signal is basically sent to an electroacoustic converter which acts as a loudspeaker and is also called an xe2x80x9cearphonexe2x80x9d. This electroacoustic earphone radiates the amplified electrical signal into the auditory canal of the pertinent ear. The auditory canal, in many cases, is sealed by an individually produced ear fitting piece (so-called xe2x80x9cotoplastyxe2x80x9d) in order to first, function as an acoustic pressure chamber which is formed by the residual volume up to the eardrum, and second, to prevent acoustic feedback between the microphone and the earphone at high degrees of amplification. Basically, there are two different designs of these hearing aids. First, in the xe2x80x9cbehind-the-ear hearing aidsxe2x80x9d (HdO), the important components of the hearing aid such as the microphone, electronic unit, battery and earphone are located in a common housing which is worn behind the ear. The amplified acoustic signal is decoupled from the earphone by a sound conduction tube and routed via the auricular muscle to the ear fitting piece and supplied through it to the auditory canal. The hearing aid can also be mounted on the frames of glasses. Second, in the xe2x80x9cIn-the-ear devicexe2x80x9d (IdO) type of hearing aid, all the aforementioned elements of the hearing aid are located in a common housing which is worn in the auricular muscle in the area of the outer auditory canal. One such in-the-ear device is integrated, for example, into the individual ear fitting piece or represents the ear fitting piece itself by a corresponding outer structure. In the in-the-ear design, the sound feed tube is eliminated since the sound exit opening is located on the side of the hearing aid facing the auditory canal and the earphone radiates the amplified acoustic signal directly into the auditory canal.
Hearing aids of the two aforementioned designs have fundamentally the following disadvantages:
The converters (earphones) of almost all hearing aids operate based on the electromagnetic conversion principle due to reasons of electrical efficiency and the optimization of the battery service life. This results in inevitable occurrence of nonlinear distortions especially at high converter currents and the pertinent output levels which adversely affect sound quality.
In addition, the first mechanical resonant frequency of this converter is generally in the middle of the spectral transmission range. This, and other physical and construction aspects, leads to an uneven frequency response and thus, undulations of the output acoustic pressure level. These resonances within the transmission range also fundamentally cause phase rotations. Both of these aspects contribute to reduced transmission quality.
The converter (earphones) are mechanically xe2x80x9copenxe2x80x9d on the output side as a result of the acoustic signal to be transmitted, thus, the outside air (except for a few cases where additional flow screens are provided) can penetrate relatively unhindered into the interior of the converter. Thus, the converter is exposed and almost unprotected to all weather and environmental effects, especially atmospheric humidity. These environmental effects are to a largely responsible for frequently occurring performance reductions of the converter operating parameters or even the failure of this component.
Especially in the in-the-ear devices, as a result of the local arrangement of the earphone in the outer (for maximally miniaturized devices) or inner auditory canal, fouling of the acoustic access channel by ear wax which is the product of the natural cleaning process of the auditory canal leads to adverse effects or failures of the earphone and thus, the hearing aid.
The primary object of this invention is to minimize or eliminate the aforementioned defects of known prior art hearing aid converters.
In accordance with one embodiment of the present invention, this and other objects and advantages are achieved by providing an electroacoustic converter for hearing aids including an electromechanical converter drive unit, a hermetically sealed metallic converter housing for enclosing the drive unit, the converter housing including one wall which is made as a bendable converter membrane, where the output-side of the converter drive unit which vibrates mechanically is coupled to the converter membrane in a manner that the converter membrane is excited in to bending vibrations thereby resulting in sound emission outside of the converter housing. The converter membrane acts as an earphone membrane which radiates sound outside the converter. The electromechanical converter drive unit within the converter may be based and operate on all known converter principles, especially piezoelectric, dielectric, electromagnetic, electrodynamic and magnetostrictive converter principles.
The converter housing is preferably cylindrical, especially circularly cylindrical, and may have a housing part which is open on one side, the open side being hermetically sealed gas tight by the converter membrane.
The housing part and/or the converter membrane can be made of a corrosion resistant, stainless metal, such as high grade steel or other body-compatible metal such as titanium, platinum, niobium, tantalum or their alloys.
Preferably, the housing part is provided with at least one single-pole, a hermetically sealed electrical housing feed through and the ground potential lying on the housing part. The housing feed through can be advantageously provided using metal-ceramic connections soldered gas tight with aluminum oxide ceramic as the insulator and at least one platinum-iridium wire as the electrical feed through lead.
The electromechanical converter drive unit is preferably a piezoelectric ceramic wafer which can be made circular and applied to the inside of the converter membrane as an electromechanically active element which, together with the converter membrane, represents an electromechanically active heteromorph composite element. Here, as in a bimorph element, the piezoelectric transverse effect is used except that the partner of the composite here does not consist of a second piezoelectrically active element, but instead, consists of the passive converter membrane of geometry similar to the piezoelement. The piezoelectric ceramic wafer can be provided with a very thin, electrically conductive coating on both sides which is used as the electrode surface and can consist especially of lead zirconate titanate. If an electrical field is applied to the piezoelectric ceramic wafer, the wafer changes its geometry, preferably in the radial direction, as a result of the transverse piezoeffect. Since extension or radial shortening is prevented by the mechanically strong connection to the passive converter membrane, sagging of the composite element takes place which is maximum in the middle with the corresponding edge support of the converter membrane.
The thickness of the converter membrane and the thickness of the piezoelectric ceramic wafer may be roughly the same and may be in the range of 0.05 mm to 0.15 mm. Furthermore, the converter membrane and the piezoelectric ceramic wafer may have roughly the same E-modulus. One especially simple and reliable structure is obtained when both the converter membrane and the housing part are electrically conductive, the piezoelectric ceramic wafer being connected electrically conductively to the converter membrane by an electrically conductive cement and the housing part forming one of at least two electric converter terminals. The radius of the converter membrane is preferably larger by a factor of 1.2 to 2.0, preferably a factor of roughly 1.4, than the radius of the piezoelectric ceramic wafer.
According to one modified embodiment of the present invention, the electromechanical converter drive unit is an electromagnet arrangement which has a component which is fixed with reference to the converter housing and a vibratory component which is coupled to the inside of the converter membrane. By using the electromagnetic converter principle, a converter frequency response, which is especially favorable for the low frequencies of the hearing range, can be achieved so that an adequate hearing impression is enabled with a sufficient loudness level using low electrical voltages.
The vibratory component of the electromagnet arrangement is preferably attached substantially in the center of the converter membrane. In particular, a permanent magnet which forms the vibratory component can be attached to the inside of the converter membrane while an electromagnetic coil operable to cause the permanent magnet to vibrate is permanently attached in the converter housing. The permanent magnet may be made as a magnetic pin and the coil can be a ring coil with a middle opening into which the magnetic pin is movably disposed. In this way, a converter arrangement with an especially small moving mass is obtained which can promptly and faithfully follow the changes of the electrical signal applied to the magnetic coil. However, it is also possible to attach the magnetic coil to the vibratory membrane and to fix the magnet with respect to the converter housing instead.
Regardless of the converter principle used in a particular application of the converter, by selecting the mechanical properties of the converter membrane and the converter drive unit, the vibratory system which encompasses these components is tuned such that the first mechanical resonant frequency of the entire converter is spectrally at the top end of the transmission range, advantageously in the range from 4 to 12 kHz and preferably, roughly 10 kHz. The converter drive unit may be electrically triggered such that the deflection of the converter membrane is impressed independently of frequency as far as the first resonant frequency.
In addition, a converter driver can also be accommodated in the converter housing.
The electroacoustic converter in accordance with the present invention may be also used in a hearing aid which has the electroacoustic converter of the above described type as the output-side acoustic converter. Such a hearing aid can be made as a behind-the-ear device, in-the-ear device, or a glasses device.
Regardless of the hearing aid type, the electroacoustic converter together with a microphone, a power supply source, signal-processing and amplifying elements and all other possible components necessary for a hearing aid function can be accommodated in a hearing aid housing.
Likewise, regardless of the hearing aid type, the electroacoustic converter of the present invention can be accommodated in a separate housing and by at least one two-pole electrical line, be connected to the actual hearing aid which contains in the conventional manner a microphone, a power supply source, signal-processing and amplifying elements and all other possible components necessary for a hearing aid to function. Here, the separate housing which contains the electroacoustic converter can be advantageously integrated into an ear fitting piece. The ear fitting piece which contains the electroacoustic converter can be mechanically connected to a behind-the-ear hearing aid via a flexibly deformable coupling element which allows individual matching to the anatomy of the outer ear and contains the electrical feed line to the converter.
When the electroacoustic converter is installed in the ear fitting piece or directly in an in-the-ear device, the converter housing is advantageously arranged such that the converter membrane ends almost flush with the area of the ear fitting piece or the in-the-ear device housing which faces the auditory canal.
Preferably, the hearing aid is equipped with an electronic converter driver which matches the signal processing electronics of the hearing aid to the selected electromechanical principle of the converter drive unit within the converter to the respective objectives of the output level and the frequency range accordingly. The converter driver can be integrated into the signal-processing electronics of the hearing aid or can be an independent electronic module. In the latter case, the converter driver can be accommodated in the hearing aid housing or the converter housing, or placed between the hearing aid and the electroacoustic converter. For a converter driver located outside the hearing aid housing, the electrical supply may be provided using the principle of phantom feed through a two-pole electrical connection between the hearing aid electronics and the converter driver, the DC voltage which supplies the converter driver being superimposed on a signal-containing AC voltage. The converter driver can also be connected via detachable mechanical or electrical plug connections to the hearing aid or the electroacoustic converter.
The converter driver may also have an integrating function for connection with a pulsewidth modulated output stage in a fully digital hearing aid having a pulse-width modulated output stage.